Uni-directional valves, also called non-return valves, are used for controlling the airflow in an air path of a ventilation device such as a filtering facemask, protective clothing or air conditioning devices. Unidirectional valves allow the air to flow through the valve in one direction, but prevent the air from flowing through the valve in the opposite direction. Uni-directional valves are used to improve the comfort and efficiency in particular of filtering face masks by allowing the exhaled air of the user to escape easily and by closing when the user inhales, so that the inhaled air can only flow through the filtering part of the filtering mask. Such uni-directional valves have been disclosed in, for example, the European patent application EP 0756881 A2 or European patent EP 0267428 B1. Furthermore, European patent application EP 0737489 A2 discloses a unidirectional valve which comprises a flexible valve element designed as a flap, the flap being secured to a housing of the uni-directional valve at one end and resting upon a seal ridge of the housing to close an air passage in a sealing manner. The flap comprises a free end that is lifted from the sealing ridge when a fluid is passing through the valve with a sufficient pressure. The valve element is held biased in the housing to close the air passage by being secured to the housing at the one end and by providing the sealing ridge with a concave curvature. The part of the flap contacting the concave curvature forms a sealing portion of the valve element and is lifted from the curvature if the valve element or flap is impinged by a sufficient air pressure.
European patent application EP 1 647 310 A1 relates to a filtering face mask comprising an exhalation valve. The exhalation valve includes a cup-shaped flexible valve element, the bottom of which forms a sealing portion interacting with the sealing section of the housing. The circumferential sidewall of the valve element extends essentially perpendicular to the bottom and is designed as an axial flexible bellow that is interacting with the housing. In order to open the exhalation valve the bottom of the flexible valve element has to be lifted from the sealing section of the housing counter to the inherent elasticity of the bellow. However, in particular the manufacturing of such a valve element is complex and cost intensive.
Ventilation devices as described above and in particular filtering face masks should be safe by not allowing contaminants to enter the interior of the face mask through the uni-directional valve. To be comfortable, the filtering face mask should displace as large a percentage of exhaled air as possible through the uni-directional valve with minimal effort.